A cathode ray tube generates an image-producing raster by means of a pair of deflection yokes which sweep the electron beam in vertical and horizontal coordinates respectively. Although electrostatic deflection is known, most yokes deflect the beam magnetically. Typically they do this by driving a sawtooth current through a deflection coil to generate a varying magnetic field which deflects the beam across the tube face.
In order to center the raster upon the CRT screen, such magnetic deflection systems may superimpose a steady magnetic field upon the varying field in order to move the center position of the beam to the geometric center of the screen. Several ways of doing this are known.
One involves the use of permanent magnets to impose a DC magnetic bias upon the neck of the CRT. Such systems involve cumbersome mechanical means for adjusting the positions of the magnets.
Other approaches avoid these mechanical problems by generating a steady magnetic bias electrically. For example, a separate bias coil (not electrically connected to the deflection coil) may be mounted on the neck of the tube, and a constant current driven through this bias coil to generate a nonvarying magnetic bias field. The magnitude of the bias field is then easily adjusted electrically, by adjusting the level and direction of the current in the separate coil. But the provision of a separate bias coil adds to the expense of the cathode ray tube.
There is another electrical approach to the problem of beam centering, one which does not require a separate bias coil. This involves driving a constant bias current through the deflection coil itself, the constant current being superimposed upon the varying deflection current which generates the raster sweep. The advantage of the approach is that it makes the deflection coil do double duty. But in this type of system it is necessary to isolate the DC bias circuit from the AC sweep circuit. Prior art proposals for accomplishing this have envisioned the use of a transformer to isolate the bias supply from the sweep current source. The transformer adds weight and expense, and also necessitates additional components for rectifying and filtering the AC voltage which is taken from the secondary of the transformer.
The problem of centering is particularly acute in the case of multiple raster systems. One example of such a system is a common type of color projection TV set which employs a separate monochromatic picture tube for each of the three primary colors, these tubes all projecting their respective different-colored images upon a common reflector screen. In such a system the three different-colored images must all be centered with respect to each other, in order to avoid the undesirable fringing which results from non-fusion of the colors. Similar problems of multiple image alignment may arise in other contexts. In some systems a plurality of CRT's may be used to project completely different images, not simply different-colored versions of the same scene, upon a common viewing surface. For example, in a plan position indicator radar system one CRT may project an image of a map (or any other background information), while another projects a continuous image of the position of an aircraft relative to the map (or any other variable data which has position-significance relative to the background information). It is also possible that in some situations a plurality of deflection circuits may be associated with a single CRT for generating different images on a common tube face on a time-shared basis. See for example the radar plan position indicator system disclosed in U.S. Pat. No. 3,159,830 of Macauley.
In each of the above mentioned systems, or whenever multiple image rasters generated by different deflection circuits are combined into a single display for any purpose, the problem of mutual raster alignment arises.
In some multiple raster systems it may be desirable, for reasons of economy, to connect a plurality of deflection coils in a series string so that they can be driven by a common sweep amplifier. Yet at the same time the alignment problem (referred to above) makes it necessary for the series-connected deflection coils to be independently compensated for any errors in centering.
Accordingly, this invention contemplates an improved system for centering a magnetic CRT deflection circuit. The system is electrical, and therefore requires no permanent magnets for mechanical linkages. It uses the deflection coil itself for both sweep and bias, and so does not require a separate bias coil. It does not require a transformer for isolation, and thus also avoids the need for rectification and filtering of the bias supply.
The invention is best appreciated from the following discussion of several illustrative embodiments, which is to be taken in conjection with the accompanying drawings.